Fetch a URL as Markdown with a signed receipt

Signed page read. Returns the URL as clean Markdown plus an Ed25519 receipt binding the Markdown's SHA-256, the fetching provider, and the observation time, so an agent can later prove what it read. The receipt attests the provider's observation (origin_signed=false), not the page's truth. A fetch that fails is not charged. Verify the receipt against the published TWZRD receipt key. x402-paid: 0.005 USDC (5000 base units) on Solana mainnet. Path A: bind to the accepts[] entry whose extra.feePayer you intend to pay; settle-rail settlement requires that feePayer to match GET /supported.